1. Field of the Invention
The present field of the invention is related to a front opening unified pod, and more particularly, to a front opening unified pod disposed with purging valve.
2. Description of the Prior Art
At present, most storage and transport devices for wafer and photomask used in semiconductor plants meet the requirements of Standard Mechanical Interface (SMIF) system proposed by Hewlett-Packard Company. This SMIF system has been disclosed in U.S. Pat. No. 4,532,970 and U.S. Pat. No. 4,534,389. The objective of the SMIF system is to ensure that in the process of transporting and storing wafer and photomask, gas surrounding wafer and photomask is static in relation to wafer and photomask, and also to prevent particles in the clean room from entering the environment adjacent to wafer and photomask.
Therefore, containers for transportation used in SMIF system are required to be equipped with property of good air tightness to prevent gas or particles of external environment from entering the containers for transportation. And in practice, take 200 mm wafer container or 300 mm wafer container for example, in order to prevent oxidization or organic contamination from occurring on the surface of wafer, gases such as nitrogen, inert gas, and dehydrated dry gas (moisture content lower than 1%) will be further conducted into the wafer container. In view of this, at least a purging valve is disposed in the aforementioned wafer containers to ensure the gas is smoothly purged and to prevent the purged gas in the wafer containers from leaking.
Referring to FIG. 1, in which is purging valve used in wafer container disclosed in U.S. Pat. No. 7,201,276. This purging valve comprises a sleeve 100, in the interior of which is disposed with a duckbill valve body 101 consisting of two pieces of elastic flaps 102. Therefore, in the purging process, gas exerts pressure outside the flaps 102 and causes slot 103 between flaps 102 to expand for gas to smoothly enter the wafer container. When the purging is stopped or when the pressure in the interior of wafer container gradually increases, the slot 103 between flaps 102 resume to status of closure to retain gas in the interior of wafer container. The structure of the aforementioned purging valve is simple, but the design of flaps 102 and the materials used are not easily accessible. When the rigidity of material is too high, the flaps 102 do not deform easily and thus the slot 103 between flaps 102 is too small for the wafer container to be filled with gas in a short time. And when the rigidity of material is too low, the slot 103 between the flaps 102 cannot completely close and thus causes leakage of gas.